Method of riveting

ABSTRACT

A method of inserting a rivet ( 2 ) into a workpiece ( 42, 44, 46 ) comprises moving the rivet ( 2 ) and workpiece ( 42, 44, 46 ) relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece. The rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece. The speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete.

The present invention relates to a method of riveting which is ofparticular, but not exclusive, application to the automotive industry.

Self-piercing riveting (SPR) is a spot-joining technique in which aself-piercing rivet is driven, by a punch, into a layered workpiecesupported on a die. The die is shaped so that as the rivet is driveninto the workpiece towards the die, the material of the workpieceplastically deforms. This flow of workpiece material causes the annulartip of the rivet to flare outwards and remain encapsulated by an upsetannulus of the workpiece material. The flared tip of the rivetinterlocking with the upset annulus of the workpiece prevents removal ofthe rivet or separation of the layers of the workpiece.

Because SPR requires plastic flow of workpiece material to allowpenetration and upsetting of the rivet, some materials are usuallyconsidered unsuitable for this technique. For example, magnesium alloys,ultra high strength steel (UHSS) and aircraft grade aluminium are notconsidered to have sufficient ductility for conventional SPR—aconventional rivet of sufficient column strength to penetrate materialsof this hardness is too resistant to flaring to be properly upset. Asanother example, polymeric workpiece layers or those of compositematerials may crack or fracture upon contact with the rivet, rather thandeforming plastically, and this can produce a weak joint and/or onewhich is more exposed to oxidation through moisture ingress. SPR istherefore conventionally only used for materials such as standard gradesof steel and forming grade aluminium.

Solid riveting (i.e. conventional riveting) is another spot joiningtechnique. A rivet with a cylindrical shank and enlarged head isinserted into a pre-formed hole in a workpiece, so that its head abutsthe top surface of the workpiece and the shank protrudes from thelayered workpiece on the other side. The protruding end of the shank isthen upset, for instance using a hammer or press in conjunction with abucking bar, peening the end of the shank to form a radially enlargedlobe which prevents removal of the rivets or separation of the layers ofthe workpiece. One problem with solid riveting is the requirement forpre-formed holes in workpieces. This increases the complexity andduration (and thus cost) of the joining process. In addition, steps mustbe taken to hold a workpiece in position after the holes have beenformed, so as to prevent different layers (and thus the holes therein)from becoming misaligned while the rivet and associated tooling ismaneuvered into place.

Another known spot-joining technique is friction stir spot welding. Infriction stir spot welding a cylindrical punch with a shouldered probeat its tip is rotated and driven into the workpiece layers to be joined.Sliding friction between the probe and the workpiece layers causes thelayers to soften and plasticize without melting, and the rotation of theprobe displaces the material and causes the plasticized portions of thetwo layers to intermingle. When the punch is withdrawn and the workpieceallowed to cool, the intermingled plasticized portions harden andproduce a welded joint between the two layers.

Friction stir spot welding is only used to weld materials of verysimilar composition, since the above intermingled plasticized portionscan only be formed if the materials of the workpiece soften at similartemperatures. Further, some materials are unsuitable for friction stirspot welding at all, for instance those which do not soften withtemperature in the required manner (such as thermosetting polymers), orthose which undergo an alteration in mechanical properties at thetemperature required (for instance, hardened steel may be brought out oftemper in the region of a friction stir spot weld).

It is an object of the present invention to mitigate or obviate one ofthe aforesaid disadvantages, and/or to provide an improved oralternative method of riveting.

According to a first aspect of the present invention there is provided amethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete;    -   one axial end of the rivet has a tip for piercing the workpiece,        and the rivet has a substantially cylindrical shank extending        longitudinally from the tip; and    -   the shank has one or more surface irregularities.

In this or any other aspect of the invention, said movement of the rivetin a workpiece relative to one another along the longitudinal axis ofthe rivet may involve moving the rivet along its longitudinal axisrelative to the workpiece, moving the workpiece along the longitudinalaxis of the rivet relative to the rivet, or moving both relative to oneanother. Alteration of the speed of rotation or the speed of movementalong the longitudinal axis of the rivet may be an increase in thatspeed or a decrease in that speed. The speed before or after thealteration may be substantially zero. The alteration in the rotationalor axial speed of the rivet may take place while the rivet is in contactwith the workpiece, for example during the time in which it is driveninto the workpiece.

One or more of said surface irregularities may be provided on a radiallyouter shank surface.

The rivet may have a bore which runs through the tip and through atleast a portion of the shank, thereby providing a radially inner shanksurface, and one or more of said surface irregularities are provided onthe radially inner shank surface.

One or more of said surface irregularities may be elongate in shape

One or more of said elongate surface irregularities may be alignedsubstantially longitudinally.

Alternatively or in addition, one or more of said elongate surfaceirregularities may be aligned substantially circumferentially.

Alternatively or in addition, one or more of said elongate surfaceirregularities may each be substantially in the shape of a helical arc.

One or more of said surface irregularities may each take the form of aprojection.

One or more of said surface irregularities may each take the form of anopening.

The or each opening may take the form of a recess, a bore, or a flatsurface (the flat surface effectively being recessed behind thecylindrical outer surface of the remainder of the rivet shank).

Where a rivet comprises one or more of said openings, one or more ofthose openings may extend between the radially inner shank surface andthe radially outer shank surface.

A rivet comprising more than one surface irregularities may compriseirregularities of different forms. For instance, it may comprise anaxial array of annular grooves and a circumferential array oflongitudinal grooves, providing the shank with a knurled surface. Asanother example, a rivet may comprise one projection and three openings.

According to a second aspect of the present invention there is providedmethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   one axial end of the rivet has a circumferentially discontinuous        tip for piercing the workpiece, and the rivet has a        substantially cylindrical shank which extends longitudinally        from the tip and provides a radially outer shank surface;

A circumferentially discontinuous tip may be considered to be present ifin a plane which is normal to the longitudinal axis of the rivet andwhich intersects the most axially distal point on the tip, at at leastone radial distance from the rivet longitudinal axis the tip intersectsthe plane at one angular position, and does not intersect the plane atanother angular position. In other words, a tip may be considered to becircumferentially discontinuous if in the above plane at least a radialportion of the tip is not circular or annular in shape. For example, astar-shaped tip may be considered to be circumferentially discontinuousin that its radially outer portion is not annular in shape, since atlarge radial distances from the longitudinal axis the tip wouldintersect the plane at the angular positions of the points on the starbut not at the angular positions of the spaces between the points. Asanother example, a tip in the shape of a rifled gun barrel may beconsidered to be circumferentially discontinuous in that its radiallyinner portion (i.e. the portion in which the ‘rifling’ is cut) is notannular in shape, since at small radial distances from the longitudinalaxis the tip would intersect the plane at the angular position of thelands of the ‘rifling’ but not at the angular positions of the groovesof the ‘rifling’.

The circumferentially discontinuous tip may be provided by one or moresurface irregularities of the form described in relation to the firstaspect of the invention.

The circumferentially discontinuous tip may comprise a plurality ofteeth.

The tip may be circumferentially discontinuous across its entire radialextent.

A rivet may be considered to be circumferentially discontinuous acrossits entire radial extent if where in a plane which is normal to thelongitudinal axis of the rivet and which intersects the most axiallydistal point on the tip, at all least one radial distance from the rivetlongitudinal axis the tip intersects the plane at one angular position,and does not intersect the plane at another angular position. In otherwords, a tip may be considered to be circumferentially discontinuousacross its entire radial extent if in the above plane no radial portionof the tip is circular or annular in shape. For example, a star-shapedtip may not be considered to be circumferentially discontinuous acrossits entire radial extent because its hub portion may be considered to beannular in shape, since at small radial distances from the longitudinalaxis the tip would intersect the plane at all angular positions of thepoints on the. As another example, a tip in the shape of a rifled gunbarrel may not be considered to be circumferentially discontinuous inthat its radially outer portion annular in shape, since at large radialdistances from the longitudinal axis the tip would intersect the planeat every angular position. A crescent-shaped tip, for example, may beconsidered to be circumferentially discontinuous across its entireradial extent. A tip with a cutting rim which is undulating orcrenelated may also be considered to be circumferentially discontinuousacross its entire axial width.

The rivet may have a bore which runs through the tip and through atleast a portion of the shank, thereby providing a substantially tubularshank portion with a radially inner shank surface.

Where the rivet has such a bore, the tip may be provided bysubstantially longitudinal slots in the substantially tubular portion.

The slots may extend between the radially inner shank surface and theradially outer shank surface.

Alternatively, the slots may be provided in the radially outer surfaceor radially inner surface of the shank but not penetrate the fullthickness of the shank wall.

The tip may have an internal taper surface which intersects the radiallyinner shank surface.

Alternatively or in addition, the tip may have an external taper surfacewhich intersects the radially outer shank surface.

Where the tip does not have a bore or concavity, the external tapersurface may taper to a point (which may or may not be intersected by thelongitudinal axis).

The taper surface may intersect said surface at an angle of less than 50degrees, for instance less than 40 degrees or less than 30 degrees.

Where the tip has an internal and/or external taper surface, the or eachtaper surface may be faceted.

Alternatively, the taper surface(s) may be smooth, textured (forinstance by knurling), undulating, or have any other suitable shape.

The tip may define a plane which is not perpendicular to thelongitudinal axis of the rivet.

For example, the plane may be positioned at an angle of at least 1degree, for instance at least 5 degrees or at least 10 degrees, to thelongitudinal axis. Alternatively or in addition, the plane may bepositioned at an angle of less than 40 degrees, for instance less than30 degrees or less than 20 degrees, to the longitudinal axis.

Such a tip is circumferentially discontinuous in that the tip wouldintersect the plane described above at a single point.

According to a third aspect of the present invention there is provided amethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   the workpiece comprises a layer made of magnesium, aircraft        aluminium, ultra-high strength steel, titanium, or metal matrix        composite.

Ultra-high strength steel may be considered to be steel with an ultimatetensile strength above around 1000 MPa, or steel with an elongationpercentage below around 12.5%. Aircraft grade aluminium may beconsidered to be aluminium with an elongation percentage below around12.5%. Aircraft grade aluminium may for instance be 7000 series or 2000series aluminium.

The workpiece may comprises a further layer, made from magnesium,aircraft aluminium, ultra-high strength steel, titanium, metal matrixcomposite, carbon fibre composite or a polymer.

Instead or as well, the workpiece may comprise an additional layer madefrom standard grade steel or forming grade aluminium, and the rivet maybe inserted into the workpiece whereby the additional layer is the finallayer contacted by the rivet.

Standard grade steel may be considered to be steel with an ultimatetensile strength below around 1000 MPa, or steel with an elongationpercentage above around 12.5%. Forming grade aluminium may be consideredto be aluminium with an elongation percentage above around 12.5%.

According to a fourth aspect of the present invention there is providedmethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   the rivet is made from aluminium, stainless steel, titanium or a        ceramic.

The rivet being made from one of these materials may allow the rivet,and a joint formed therewith, to be more resistant to corrosion (such asoxidation).

According to a fifth aspect of the present invention there is provided amethod of inserting a rivet into a workpiece using a riveting tool, themethod comprising moving the rivet and workpiece relative to oneanother, along a longitudinal axis of the rivet, so as to drive therivet into the workpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, by the riveting tool for at least part of the        time during which it is in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   the riveting tool drives the rivet to rotate though one or more        rotary drive components in frictional engagement with the rivet.

The riveting tool may comprise a tool nose and a punch reciprocallydisposed therein, the punch providing axial force to the rivet so as todrive it into the workpiece.

Where the relative movement of the workpiece and rivet along thelongitudinal axis of the rivet takes place by moving the workpiece, theaxial force from the punch may be a reaction force.

The or one of said rotary drive components may be the punch.

Instead or as well, the or one of said rotary drive components may bethe tool nose.

Where the tool nose is a rotary drive component, the riveting tool maycomprise a pressure surface which is rotatably mounted to the nose, thepressure surface contacting the workpiece during insertion of the rivetand rotating relative to the nose for at least part of the time it is incontact with the workpiece.

According to a sixth aspect of the present invention there is provided amethod of inserting a rivet into a workpiece using a riveting tool, themethod comprising moving the rivet and workpiece relative to oneanother, along a longitudinal axis of the rivet, so as to drive therivet into the workpiece, wherein:

-   -   wherein the riveting tool comprises a tool nose and a punch        reciprocally disposed therein;    -   the punch provides axial force to the rivet so as to drive it        into the workpiece;    -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, by the nose of the riveting tool for at least        part of the time during which it is in contact with the        workpiece; and    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete.

The punch may not rotate relative to the workpiece during driving of therivet into the workpiece

Alternatively, the punch may rotate along with the rivet. For instance,the punch may be rotatable so as to allow it to freewheel, or the toolnose may be in driving engagement with the punch.

The punch rotating along with the rivet may be beneficial in that it mayreduce the speed of wear experienced by the punch at the interfacebetween the punch and a rotating surface (for example a surface of therivet or of another component of the riveting tool).

According to a seventh aspect of the present invention there is provideda method of inserting a rivet into a workpiece using a riveting tool,the method comprising moving the rivet and workpiece relative to oneanother, along a longitudinal axis of the rivet, so as to drive therivet into the workpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, by the riveting tool for at least part of the        time during which it is in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   the riveting tool drives the rivet to rotate though a rotary        drive component which engages with the rivet about the        circumferential periphery of a portion of the rivet.

The rivet may have a tip for piercing the workpiece at one axial end, ashank extending longitudinally from the tip, and a head extendingradially outwards from the shank.

Where the rivet has a head, the portion of the rivet engaged by therotary drive component may include a radially peripheral edge or surfacedefined by the head. Instead or as well, the portion of the rivetengaged by the rotary drive component may include a fillet or chamfer atthe intersection between the head and the shank.

According to an eighth aspect of the present invention there is provideda method of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   the rivet has a longitudinal bore which extends along its entire        axial length.

The rivet may be rotated by a rotary drive component of a riveting tool,the rotary drive component engaging with a section of the bore which hasa non-circular cross section. Said portion of the bore may be, forexample, square, hexagonal or ovoid in cross section.

The rotary drive component may be a punch which provides axial force tothe rivet so as to drive it into the workpiece, the punch engaging withsaid section of the bore through a complementarily-shaped driving bitprojecting therefrom.

The driving bit may be movable between an extended position in which itprojects from a distal surface of the punch, to a retracted position inwhich it projects from said distal surface of the punch to a reducedextent or is flush with said distal surface of the punch.

The rivet may be substantially symmetrical along its longitudinal axis.

The punch may have a profiled tip which applies said axial force to oneaxial end of the rivet, and deforms that end of the rivet during drivingof the rivet into the workpiece.

According to a ninth aspect of the present invention there is provided amethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete;    -   the rivet has a tip for piercing the workpiece at one axial end,        a shank extending longitudinally from the tip, and a head        extending radially outwards from the shank;    -   the head defines an underside which faces towards the tip; and    -   the rivet has a cavity provided in the underside of the head, or        in a portion of the shank adjacent thereto, within which        workpiece material may be accommodated.

Where such a cavity is provided in the shank, it may constitute asurface irregularity according to the first aspect of the invention.

Workpiece material may enter the cavity during insertion of the rivetinto the workpiece.

Alternatively, the cavity may be provided to guard againsteventualities, workpiece material remaining clear of the cavity when themethod is performed correctly.

According to a tenth aspect of the present invention there is provided amethod of inserting a rivet into a workpiece, the method comprisingmoving the rivet and workpiece relative to one another, along alongitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete;    -   the rivet has a tip for piercing the workpiece at one axial end,        a shank extending longitudinally from the tip, and a head        extending radially outwards from the shank; and    -   a portion of the shank at the end of the rivet nearest the head        has a larger diameter than the remainder of the shank.

Said portion of the shank may constitute an annular circumferentialprojection according to the first aspect of the invention.

Said portion of the shank may be substantially cylindrical.

Alternatively, said portion of the shank may be frustoconical.

According to an eleventh aspect of the present invention there isprovided a method of inserting a rivet into a workpiece, the methodcomprising moving the rivet and workpiece relative to one another, alonga longitudinal axis of the rivet, so as to drive the rivet into theworkpiece, wherein:

-   -   the rivet is rotated about its longitudinal axis, relative to        the workpiece, for at least part of the time during which it is        in contact with the workpiece;    -   the speed of said rotation, or the speed of movement along the        longitudinal axis of the rivet, is altered at least once before        driving of the rivet into the workpiece is complete; and    -   auxiliary heating is applied to the workpiece and/or the rivet        at at least one point before, during or after driving of the        rivet into the workpiece.

Auxiliary heating may be utilised so as to soften workpiece materialinstead of or as well as frictional heating through contact with therivet.

The auxiliary heating may be provided at least in part by a laser beam.

Instead or as well, the auxiliary heating may be provided at least inpart by ultrasonic energy.

Where the auxiliary heating is provided at least in part by ultrasonicenergy, the rivet may be is driven into the workpiece towards a die, andat least part of said ultrasonic energy may be applied to the workpieceby the die. Alternatively or in addition, ultrasonic energy may beapplied to the workpiece and/or the rivet by a tool nose or a punch.

In any aspect of the present invention, the rivet may pierce a firstworkpiece layer at a first rotational speed, before piercing a secondworkpiece layer at a second rotational speed.

The first rotational speed may be higher than the second rotationalspeed.

Alternatively, the first rotational speed may be lower than the secondrotational speed.

The rivet may pierce a first workpiece layer at a first axial speed,before piercing a second workpiece layer at a second axial speed.

Where a rivet pierces a first workpiece layer at a first rotationalspeed before piercing a second workpiece layer at a second rotationalspeed, and pierces a first workpiece layer at a first axial speed beforepiercing a second workpiece layer at a second axial speed, the firstworkpiece pierced at the first rotational speed may or may not be thesame layer as the first workpiece layer pierced at the first axialspeed. Similarly, the second workpiece pierced at the second rotationalspeed may or may not be the same layer as the second workpiece layerpierced at the second axial speed.

The first axial speed may be higher than the second axial speed.

Alternatively, the first axial speed may be lower than the second axialspeed.

The rivet may penetrate at least a portion of the workpiece with arotational speed of substantially zero.

Substantially zero may be considered to mean zero sufficiently low tohave negligible effect on the behaviour of the rivet and the workpiece.

The speed of substantially zero may or may not be the first rotationalspeed or the second rotational speed described above.

The speed of rotation of the rivet may be altered at least twice beforedriving of the rivet into the workpiece is complete.

The axial speed of the rivet may be altered at least twice beforedriving of the rivet into the workpiece is complete.

The (axial or rotational) speed being altered at least twice is notintended to be limited to the rivet moving at three differentaxial/rotational speeds. For example, the rotational or axial speed ofthe rivet may be altered once, then altered a second time to bring itsspeed back to its initial value.

The or at least one of the alterations in axial or rotational speed ofthe rivet may be brought about by resistive forces applied to the rivetby the workpiece.

For example, the speed of the rivet may decay as the depth to which haspenetrated the workpiece increases (as increased depth of penetrationmay result in increased resistive forces), irrespective of the materialcomposition of the layers of the workpiece, or as the rivet contacts adifferent layer of the workpiece (which may be harder, and thus provideincreased resistive force).

The or at least one of the alterations in speed of rotation of the rivetmay be brought about by an alteration in the magnitude of the torqueapplied to the rivet so as to cause it to rotate.

The or at least one of the alterations in axial speed of the rivet maybe brought about by an alteration in the magnitude of axial forceapplied to the rivet or workpiece so as to drive the rivet into theworkpiece.

The axial force applied to drive the rivet into the workpiece may besubstantially constant.

The or at least one of the alterations in axial or rotational speed ofthe rivet may occur as a result of the rivet contacting different layerswithin the workpiece.

The alterations in speed may be due to resistive forces as describedabove, or as a result of active control. For example, a riveting toolinserting the rivet may detect the position of the rivet within theworkpiece and alter its speed in response.

The axial movement of the rivet relative to the workpiece may be pausedat at least one point before driving of the rivet into the workpiece iscomplete. For example, the axial movement of the rivet relative to theworkpiece may be paused when the rivet first contacts a layer of theworkpiece. Said layer may be the top layer of the workpiece, at whichpoint the axial movement of the rivet relative to the workpiece would bepaused when the rivet first contacts the workpiece.

According to a twelfth aspect of the present invention there is provideda method of manufacturing a product, the method comprising fasteningtogether two or more layers of a workpiece using a method according toany preceding claim.

According to a thirteenth aspect of the present invention there isprovided a product manufactured using a method according to the twelfthaspect of the invention.

In the eleventh or twelfth aspects of the invention the product may be avehicle, such as a motorcycle, car, van, lorry or aircraft.

According to a fourteenth aspect of the present invention there isprovided a rivet having the features of any of the first, second,fourth, eighth, ninth or tenth aspects of the invention.

Aspects of the invention may be particularly suited for use withworkpieces which have one or more layers with a ductility below around12.5% elongation, for instance below around 10% elongation. For example,aspects of the invention may be particularly suited for use withworkpieces formed from magnesium (magnesium may have an elongationpercentage of around 8%). Aspects of the invention may also beparticularly suited for use with workpieces which have one or morelayers with an ultimate tensile strength above around 1000 MPa, forinstance above around 1,200 MPa.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a cross-sectional side view of a conventional SPR rivet;

FIG. 2 is a cross-sectional side view of another conventional SPR rivet;

FIG. 3 is a cross-sectional side view of a further conventional SPRrivet;

FIG. 4 is a series of schematic cross-sectional side views of stages ina method according to a first embodiment of the invention;

FIG. 5 is a schematic cross-sectional side view of part of the apparatusused in a method according to a second embodiment of the invention;

FIG. 6 is a schematic cross-sectional side view of part of the apparatusused in a method according to a third embodiment of the invention;

FIG. 7 is a series of schematic cross-sectional side views of stages ina method according to a fourth embodiment of the invention;

FIG. 8 is a side view of a rivet suitable for use with the invention;

FIG. 9 is a side view of another rivet suitable for use with theinvention;

FIG. 10 is a side view of a further rivet suitable for use with theinvention;

FIG. 11 is a perspective view of an additional rivet suitable for usewith the invention;

FIG. 12 is a side view of another rivet suitable for use with theinvention;

FIG. 13 is a series of schematic side views of a rivet suitable for usewith the invention, part of a tool suitable for inserting the rivet, anda joint produced using the rivet;

FIG. 14 is a side view of another rivet suitable for use with theinvention;

FIG. 15 is a cross-sectional side view of a further rivet suitable foruse with the invention;

FIG. 16 is a cutaway view of part of a rivet suitable for use with theinvention;

FIG. 17 is a side view of an additional rivet suitable for use with theinvention;

FIG. 18 is a perspective view of a rivet suitable for use with theinvention;

FIG. 19 is a perspective view of another rivet suitable for use with theinvention;

FIG. 20 is a perspective view of a further rivet suitable for use withthe invention;

FIG. 21 is a cross-sectional side view of an additional rivet suitablefor use with the invention;

FIG. 22 is a perspective view of a rivet suitable for use with theinvention;

FIG. 23 is a cross-sectional side view of another rivet suitable for usewith the invention;

FIG. 24 is a series of schematic side views of a rivet suitable for usewith the invention, and part of a tool suitable for inserting the rivet;

FIG. 25 is a cross-sectional side view of a further rivet suitable foruse with the invention;

FIG. 26 is a series of schematic side views of a further rivet suitablefor use with the invention, and part of a tool suitable for insertingthe rivet;

FIG. 27 is a side view of a further rivet suitable for use with theinvention;

FIG. 28 is a side view of another rivet suitable for use with theinvention;

FIG. 29 is a side view of a further rivet suitable for use with theinvention;

FIG. 30 is a side view of an additional rivet suitable for use with theinvention; and

FIG. 31 is a side view of another rivet suitable for use with theinvention.

Referring now to the drawings, FIG. 1 shows a conventional self-piercingrivet 2. The rivet 2 defines a longitudinal axis 4, with a tip 6 at oneaxial end, a head 8 at the other axial end, and a substantiallycylindrical shank 10 extending therebetween. The rivet also has a bore12 which is substantially coaxial with the rivet longitudinal axis 4. Inthis example the bore 12 runs through the tip 6 and along the entirelength of the shank 10. The shank 10 therefore defines a radially outersurface 14 and a radially inner surface 16, each of which issubstantially cylindrical in shape (and positioned substantiallycircumferentially about the longitudinal axis 4). The tip 6 of the rivet2 has an internal taper surface 18 which provides the tip 6 with anannular cutting rim 20. In this example, the internal taper surface 18intersects the radially inner surface 16 of the shank 10 at an angle ofaround 140°. Further, the internal taper surface 18 extends sufficientlyradially outwards that that the cutting rim 20 of this example may beconsidered to be a substantially circumferential cutting edge at theintersection of the taper surface 18 and the radially outer surface 14of the shank.

The head 8 of the rivet 2 defines a circumferential outer periphery 22,which in this case takes the form of a substantially cylindrical surfacepositioned substantially circumferentially about the rivet longitudinalaxis 4. The head 8 also defines an underside 24, on the side of the headnearest the tip 6 of the rivet 2. In this example, the underside 24takes the form of a frustoconical surface positioned substantiallycircumferentially about the rivet longitudinal axis, which meets theradially outer surface 14 of the shank 10 at a filleted intersection 26.The head 8 further defines a pressure surface 28 upon which axial forcecan be applied to drive the rivet 2 into a workpiece (not visible).

In this example the bore 12 extends through the entire axial length ofthe shank 10, therefore the shank is substantially tubular. In othercases, however, the bore 12 may extend only part way along the shank 10.In such cases, the portion of the shank 10 through which the bore 12runs may be referred to as the substantially tubular portion of theshank.

FIG. 2 shows another conventional SPR rivet 2, which is similar to thatshown in FIG. 1. In this example, however, the tip 6 of the rivet 2 hasan external taper surface 30 rather than an internal taper surface 18 asfound in the rivet of FIG. 1. The external taper surface 30 intersectsthe radially outer surface 14 of the shank 10 (at an angle of around140° in this case).

Further, although the cutting rim 20 of the rivet of FIG. 1 is a sharpedge, the cutting rim 20 of rivet 2 of FIG. 2 takes the form of anannular surface positioned substantially circumferentially about therivet longitudinal axis 4. Further, in this example the underside 24 ofthe head 8 of the rivet 2 is defined entirely by the filletedintersection 26. Also, the circumferential outer periphery 22 of thehead 8 is defined by an annular edge rather than a cylindrical surface.

FIG. 3 shows a further conventional SPR rivet 2. Unlike the rivets ofFIGS. 1 and 2 the bore 12 has a tapered tip 32, rather than a roundedtip. In addition, the underside 24 of the head 8 takes the form of anannular surface positioned substantially circumferentially around therivet longitudinal axis 4, and the junction between the underside of thehead and the radially outer surface of the shank 10 does not have afilleted intersection. Furthermore, the tip 6 of the rivet 2 does nothave a taper surface (18, 30 in FIGS. 1 and 2 respectively). Instead,the cutting rim 20 takes the form of an annular surface, positionedsubstantially circumferentially about the rivet longitudinal axis 4,extending between the radially outer and inner surfaces 14, 16 of theshank 10.

A method according to a first embodiment of the invention will now bedescribed with reference to FIG. 4, which illustrates the stages of themethod as FIGS. 4A to 4E. This embodiment is part of the productionprocess for a motor vehicle (although it could also be used in theproduction of other products). In the first embodiment the workpiece 46has a first layer 42 which is part of a vehicle chassis made ofmagnesium alloy, and a second layer 44 which is part of a vehiclebodywork panel made of conventional forming grade aluminium, which arejoined together by inserting a rivet 2 of the type shown in FIG. 1. Inthis case, the rivet 2 is made out of titanium. In this embodiment thesecond layer 44 is an example of an ‘additional layer’. The rivet 2 isinserted using a riveting tool that comprises a punch 48 reciprocallyreceived within a tool nose 50, and a die 52. In this embodiment thetool nose 50 is substantially tubular and the punch 48 is substantiallycylindrical, and the tool nose and punch are substantially coaxiallyarranged relative to one another. The die 52 has a pip 54 and an annularcavity 56. The pip 54 and cavity 56 are also substantially coaxial withrespect one another. The tool nose 50 is resiliently connected to thepunch 48 so that if the punch is moved axially the tool nose tends to gowith it, but if the tool nose is prevented from moving then the punchcan continue its motion. The tool nose 50 of this embodiment also has atemperature sensor 57 at its distal tip.

In the description below, the ‘top’ of the workpiece is the portion ofthe workpiece which the rivet contacts first, and the ‘bottom’ of theworkpiece is the portion which contacts the rivet last (i.e. whichcontacts the die, in this case), irrespective of the spatial orientationof the workpiece relative to the ground. For instance, the rivet may beinserted into the workpiece from underneath, but will nonetheless firstcontact the ‘top’ layer.

To join the layers 42, 44 of the workpiece 46, the workpiece ispositioned on an annular support surface 57 of the die 52, and a rivet 2is mounted within the tool nose 50. More specifically, in this case therivet 2 is mounted such that its pressure surface 28 abuts a distalsurface 58 of the punch 48, and is positioned so as to be substantiallycoaxial with the tool nose 50 and punch 48. The tool nose 50 (along withthe punch 48 and rivet 2) is positioned opposite the die 52. The toolnose 50, punch 48, and rivet 2 are thus all substantially coaxial withthe pip 54 and cavity 56 of the die 52. The punch is advanced along itslongitudinal axis (which is also the longitudinal axis of the tool nose50 and the rivet 2) towards the workpiece 46 by an actuator in the formof a hydraulic cylinder (not visible), and the tool nose moves axiallywith it. When the tool nose 50 contacts the top of the workpiece 46 itcan move no further, but the punch continues to move towards theworkpiece. As a result of the resilient connection between the punch 48and tool nose 50, the continued motion of the punch urges the tool noseagainst the workpiece 46. The workpiece is therefore held in positionbetween the die 52 and the distal end of the tool nose 50. Since thedistal surface 58 of the punch 48 is in contact with the pressuresurface 28 of the rivet 2, as the punch 48 advances axially towards theworkpiece within the tool nose 50, the rivet advances as well. This isshown in FIG. 4A. The punch and rivet are advanced towards theworkpiece, for example, at an axial speed of 300 mm/s.

When the rivet 2 contacts the workpiece top surface of the workpiece 46,the axial movement of the punch 48 (and thus the rivet) is paused by thecontrol unit. The punch 48 is then driven to rotate about itslongitudinal axis within the tool nose 50 by a motor while the rivet isin contact with the workpiece 46. Due to friction between the distalsurface 58 of the punch 48 and the pressure surface 28 of the rivet 2,as the punch rotates the rivet rotates with it. The punch 48 thereforefunctions as a rotary drive component in frictional engagement with therivet 2. The punch is connected to a control unit (not visible) by arotary positional encoder (not visible). Through the encoder, thecontrol unit monitors the speed of the punch and adjusts the speed ofthe motor (not visible) so that the punch rotates at a constant speed,for example, in this case 6,000 RPM.

As the rivet 2 rotates about its longitudinal axis on the surface of theworkpiece 46, as shown in FIG. 4B, the heat generated by the slidingfriction between the rivet and the workpiece softens the workpiece inthe locality of the rivet. In particular, it softens the magnesium ofthe first layer 42 so that it becomes sufficiently ductile to bepenetrated by the rivet 2 without premature deformation of the rivet (orcracking of that layer). The control unit monitors the temperature ofthe top of the workpiece 46 using the sensor 57. Once the temperaturehas increased sufficiently, the control unit alters the speed ofrotation of the rivet 2 by making an adjustment to the speed at whichthe punch 48 is rotated by the motor. More particularly, the speed ofthe punch 48, and thus of the rivet 2, is reduced, in this example, to500 RPM. The control unit then resumes the axial advance of the punch 48and rivet 2 under action of the hydraulic cylinder (not visible). Atthis point, the rivet 2 travels, for example, at an axial speed of 200mm/s.

Continued movement of the rivet 2 relative to the workpiece 46 causesthe rivet to begin penetrating the top of the workpiece (i.e. the firstlayer 42), as shown in FIG. 4C. As the rivet 2 penetrates the workpiece46, some workpiece material 60 at the top of the workpiece is forced upinto the bore 12 of the rivet 2. Further, the axial force applied by therivet 2 to the workpiece 46 causes some workpiece material 62 to beforced into the cavity 56 of the die 52.

The rivet 2 continues to penetrate the workpiece 46 while rotating at500 RPM. This speed is sufficiently low that negligible additionalfrictional heating is supplied, but rotation of the rivet 2 as ittravels axially may reduce the extent to which workpiece material isliable to stick to the rivet and be dragged down into the workpiece 46by the rivet. This may be desirable since workpiece material beingdragged down with the rivet can lead to the top surface of the workpiecehaving a significant indentation in the region surrounding the rivet,which may be undesirable aesthetically or may provide greateropportunity for moisture ingress (which may, in turn, lead to oxidationin the region of the joint).

When the rivet 2 has penetrated the workpiece 46 to the point that itcontacts the second workpiece layer 44, further axial travel of therivet causes it to be upset. Workpiece material 62 from the second layer44 forced into the cavity 56 is directed radially outwards by the pip54. Since the tip 6 of the rivet 2 has penetrated into this portion ofthe workpiece, this plastic flow causes the tip to flare outwards,upsetting the rivet. Throughout insertion of the rivet 2, workpiecematerial 60 continues to travel up the bore 12 of the rivet.

When the rivet 2 reaches the point where its contact surface 28 is flushwith the top surface of the workpiece, as shown in FIG. 4D, driving ofthe rivet 2 into the workpiece is complete. The control unit (notvisible) therefore stops the rotation and the axial advance of the punch48, and thus the rivet 2 stops moving as well. In the completed joint,the workpiece material 60 which entered the bore 12 of the rivet 2 formsa ‘slug’ 64 which in this embodiment occupies substantially all thevolume of the bore, and the workpiece material 62 forced into the cavity56 of the die 52 forms an upset annulus 66 encapsulating the flared tipof the rivet (as described above).

After the joint is completed the punch 48 is retracted, leaving therivet 2 in place, as shown in FIG. 4E. At first, the tool nose 50remains urged against the workpiece due to its resilient connection tothe punch 48, but once the punch has been retracted sufficiently thetool nose 50 moves upwards with it and lifts off the surface of theworkpiece. A subsequent rivet 2 can then be mounted in the tool nose 50,and another joint (either in a different position on the workpiece 46,or on a different workpiece) can be produced.

In this embodiment, the rotational speed of 6,000 RPM may be consideredto be an example of a first rotational speed and 500 RPM an example of asecond rotational speed, in which case the first rotational speed ishigher than the second rotational speed. Other rotational speeds may beused.

In the case of this embodiment, the point at which penetration of theworkpiece 46 by the rivet 2 commences is selected by the control unit sothat the friction stir heating applied to the top layer 42 has not yetraised the temperature of the bottom layer 44 sufficiently for it to besoftened to any great extent. This is because in this embodiment thesecond workpiece layer 44 is made of forming grade aluminium, which hassuitable mechanical properties for conventional SPR. The alteration ofthe rotational speed of the rivet 2 is therefore timed so that thesecond workpiece layer 44 is largely unaffected and SPR can be performedas normal (albeit with rotation of the rivet 2 still taking place at alower speed). If the rotational speed of the rivet had continued at thespeed sufficient to soften the top layer 42 when the rivet was beinginserted, the bottom layer 44 would have been too soft to upset therivet while flowing into the cavity 56.

Although the embodiment refers to joining magnesium alloy and aluminium,the embodiment may be used to join other materials. Although rotationalspeeds of 6,000 RPM then 500 RPM are referred to above, other rotationalspeeds may be used. The rotational speeds may be selected for acombination of materials using experimental testing.

A method according to a second embodiment of the invention will now bedescribed with reference to FIG. 5. The second embodiment is similar tothe first embodiment, therefore only the differences will be describedin detail. The second embodiment is again part of the production processfor a motor vehicle (although it could be used for other products). Inthis case, the workpiece 46 has a first layer 42 which is part of aninsulation panel made out of polymer, a second layer 44 which is part ofa vehicle body work panel made of titanium alloy, and also a third layer70 which is part of a vehicle chassis component made of standard gradesteel. In this case, the first layer 42 is the top layer and the thirdlayer 70 is the bottom layer. The rivet 2 is again of the type describedin relation to FIG. 1, however in this embodiment it is made out ofstainless steel. In this embodiment either the first layer 42 or thesecond layer 44 (but not both) may be considered to constitute a‘further layer’, and the third layer 70 may be considered to constitutean ‘additional layer’.

In this embodiment, the riveting tool has a clamping washer 72 which isrotatably mounted to the tool nose 50 by bearings 74. The clampingwasher 72 has a pressure surface 76 which in this embodiment takes theform of an annular surface position substantially circumferentiallyabout the longitudinal axis of the tool nose 50 (and the punch 48). Inthis embodiment, the workpiece 46 is held in position between thesupport surface 57 of the die 52 and the pressure surface 76 of theclamping washer 72, rather than directly contacting the tool nose 50 aswas the case in the first embodiment. The second embodiment alsoincludes an auxiliary heating unit in the form of a laser 78 and a lens80 configured to spread the collimated beam 82 from the laser 78 into adivergent beam 84, as discussed below.

In this embodiment, it is the tool nose 50 rather than the punch 48which functions as the rotary drive component. Indeed, in thisembodiment the punch 48 is unable to rotate relative to the workpiece46. When a rivet 2 is mounted in the tool nose 50 prior to insertioninto a workpiece, it is inserted into a bore 86 in the tool nose 50which is sized so as to exhibit an interference fit with the rivet (inthis case with the circumferential periphery 22 of the head 8 of therivet). The interference fit between the rivet 2 and bore 86 providessubstantial frictional engagement, thereby allowing the tool nose 50 todrive the rivet 2 such that the rivet and tool nose rotate in unison. Inthis embodiment the bore is of complementary shape to thecircumferential periphery of rivet 2 (in this case the head 8 of therivet), however this need not be the case for a suitable interferencefit to be achieved. As an alternative to an interference fit the rivethead may be provided with axial ridges or grooves which engage withcomplementary features provided on the bore 86.

The laser 78 is used to heat the underside of the workpiece (i.e. thethird layer 70) before the die 52 and the tool nose 50 (along with thepunch 48 and a rivet 2) are moved into position to form the joint. Thedivergent beam 84 is directed onto the portion of the third layer 70 atwhich the riveted joint will be formed. Energy from the laser heats thethird layer 70 so as to reduce the total amount of friction stir heatingwhich must take place (as explained below). The control unit (notvisible) monitors the duration for which the laser 78 is operational, soas to indirectly monitor the temperature of the third layer 70 of theworkpiece 46. When the control unit determines that the laser 78 hasbeen operational for long enough for the third layer 70 to have reachedthe required temperature, the laser shuts off and the tool nose (withthe punch 48 and a rivet 2) and dye 52 are positioned substantiallycollinearly at the portion of the workpiece 46 at which a joint isrequired. A rivet may be mounted to the tool nose 50 before, during orafter operation of the laser (and before, during or after positioning ofthe tool nose).

With a rivet mounted within the tool nose 50, and the tool nose and die52 in position, the tool nose and punch 48 (and thus also the rivet 2)are advanced axially towards the workpiece 46 supported on the die at aspeed, for example, of 300 mm/s. In this embodiment, axial movement isproduced by an electrical linear actuator (not visible), or a hydrauliccylinder. When the tool nose 50 has advanced sufficiently for thepressure surface 76 of the clamping washer 72 to contact the top surfaceof the workpiece 46, it is held against the workpiece as describedabove. In this embodiment, the resilient connection between the toolnose 50 and the punch 48 is configured to clamp the workpiece 46 betweenthe clamping washer 72 and the die 52 during continued movement of thepunch, rather than merely holding the workpiece 46 in position. In thiscase, the clamping force applied reaches 3 kN when the punch 48 hasadvanced within the tool nose 50 sufficiently for the rivet 2 to contactthe workpiece 46. In some situations, clamping the workpiece in such amanner can improve the quality of the joint by limiting the surface areaof the workpiece 46 within which the material can deform. For instance,it may help to limit the extent to which workpiece is dragged down bythe rivet as (described above).

Once the rivet 2 contacts the workpiece 46, the axial movement of thepunch is paused. At that point, the control unit (not visible) directspower to the motor so as to cause it to apply a torque of 2 Nm to thetool nose 50 (the control unit monitoring the torque applied to the toolnose through a suitable transducer), causing the tool nose to rotate at4,000 RPM for example. This, in turn, causes the rivet 2 to rotate onthe surface of the workpiece (at the same speed) and generate frictionalheating. In addition to friction between the rivet 2 and the workpiece46, since the punch 48 does not rotate there is significant frictionalheating between the distal surface 58 of the punch and the pressuresurface 28 of the rivet 2. This heats the rivet 2, and the rivetconducts some of this heat to the workpiece 46, reducing the amount oftime taken to soften the workpiece material. Heating the rivet 2 alsoincreases its ductility, allowing it to flare sufficiently whenrequired. In contrast, in some applications without such heating thestainless steel material of the rivet may be too brittle to deformcorrectly (however in other applications a stainless steel rivet mayhave sufficient ductility at room temperature, or at the temperaturereached through frictional contact with the workpiece alone).

It is noteworthy that while the tool nose 50 rotates, the clampingwasher 72 does not. The bearings 74 by which the clamping washer 72 isconnected to the tool nose 50 limit the torque applied to the clampingwasher by the tool nose, and friction between the pressure surface 76 ofthe clamping washer and the workpiece 46 holds the clamping washer in asubstantially rotationally static position. In contrast, if the clampingwasher 72 rotated with the tool nose 50 it may burrow into the workpiecematerial itself, producing an unaesthetic and potentially weaker joint.

When the rivet 2 first begins to rotate, the resistance to rotationexerted on the rivet by friction between the rivet and workpiece 46,friction between the rivet and punch 48 and friction in the bearings 74limit the rotational speed of the tool nose 50 and rivet, in this caseto 2,000 RPM for example. However, as the top of the workpiece 46 isheated and softens, the frictional resistance between it and the rivetdecreases. The control unit monitors the speed of the tool nose 50, andwhen the speed increases to a threshold value, for instance 3,000 RPM inthis case, the punch 48 begins to push the rivet 2 into the workpiece46. In this case, the control unit regulates the actuator so that theactuator exerts a force of 2 kN to the punch 48 (the control unitmonitoring the force experienced by the punch and thus the rivet 2 usinga force transducer such as a strain gauge) so as to move the rivet intothe polymeric material of the first workpiece layer at 180 mm/s.

Once the rivet contacts the second layer 44 the frictional resistanceexperienced by the rivet 2 increases due to the titanium material of thesecond layer being significantly harder than the polymeric material ofthe first layer 42, and requiring a higher temperature to be softened.This increase in resistance causes the rotation of the rivet 2 and toolnose 50 to slow down. This change is detected by the control unit. Inresponse, the control unit pauses the axial movement of the rivet andincreases the torque applied by the motor to the tool nose 50 to 5 Nm,which in turn increases the rotational speed of the rivet 2, e.g. to5,000 RPM. The rivet 2 therefore rotates on the top surface of thesecond layer 44, allowing time for the additional friction stir heatingnecessary to soften the titanium material of this layer. Once theresistance to rotation drops again, the control unit detects theassociated rise in speed and begins driving the rivet 2 into the secondlayer 44 at an axial speed of 140 mm/s by regulating the actuator toapply an axial force of 1.5 kN.

When the rivet reaches the third layer 70, resistance to rotation of therivet 2 (and thus the tool nose 50) increases again. At this point,however, the control unit does not change the torque with which the toolnose 50 is driven. As a result, the speed of rotation of the rivetdecreases (in this case, for example, to 3,000 RPM). Since the thirdlayer has been heated by the laser 78, however (and due to therequirement for this layer to exhibit a degree of resistance todeformation so as to upset the rivet) little or no friction stir heatinput from the rivet 2 is required for the third layer 70 to be softenedsufficiently for SPR. The rivet therefore continues its axial travelinto the third layer 70 without pausing at its surface, and indeed thecontrol unit increases the speed of the rivet to, for example, 150 mm/sby increasing the axial force exerted on the punch to 3 kN. Duringpenetration of the third layer 70, workpiece material deforms into thecavity 56 of the die 52 and upsets the rivet 2 as described above.

In this embodiment, the axial speed of 180 mm/s may be considered to bea first axial speed and the axial speed of 140 mm/s a second axialspeed, in which case the first axial speed is larger than the secondaxial speed. The terms “first axial speed” and “second axial speed” areintended to distinguish between an axial speed and a subsequent axialspeed. These terms are not intended to mean that there can be no axialspeed preceding the first axial speed, nor is it intended to mean thatthere can be no axial speed between the first axial speed and the secondaxial speed. Similarly, in this embodiment the rotational speed of 4,000RPM may be considered to be a first rotational speed and the speed of2,000 RPM or the speed of 3,000 RPM may be considered to be a secondrotational speed (in which case the first rotational speed is largerthan the second rotational speed). The terms “first rotational speed”and “second rotational speed” are intended to distinguish between arotational speed and a subsequent rotational speed. These terms are notintended to mean that there can be no rotational speed preceding thefirst rotational speed, nor is it intended to mean that there can be norotational speed between the first rotational speed and the secondrotational speed.

Although it would be possible to soften the third layer 70 throughfriction stir heating alone, the rotational speed of the rivet 2 whichwould be required to do this effectively would have an adverse effect onthe first layer 42. For example, the rivet spinning at such speeds mayheat the first layer 42 too much, causing the workpiece material of thislayer to melt rather than soften (at which point it could spray outunder centrifugal force and leave insufficient material in the region ofthe joint), or catch light. This embodiment therefore reduces therotational speed of the rivet after penetration of the second layer 44,so as to minimize the risk of any such adverse consequences, with theadditional heating required by the third layer 70 being supplied by thelaser 78.

Although this embodiment refers to joining a particular combination ofmaterials, the embodiment may be used to joint other combinations ofmaterials. Appropriate rotational speeds may be determinedexperimentally for different combinations of materials.

A method according to a third embodiment of the invention will now bedescribed with reference to FIG. 6. The third embodiment is also similarto the first embodiment, therefore only the differences will bediscussed here. In this embodiment the first layer 42 is part of avehicle bodywork panel made out of carbon fibre composite, and thesecond layer 46 is part of a vehicle chassis made out of forming gradealuminium. The embodiment (like any other embodiment) may also be usedfor other materials and/or other products, or the rivet may be made froma different suitable material (such as stainless or non-stainless steel,magnesium, titanium or a different grade of aluminium).

In this embodiment, the tool nose 50 is a rotary drive component inengagement with the rivet 2 (e.g. due to friction or complementary axialfeatures which engage each other). In this case, the tool nose has asubstantially circumferential array of four retractable clamp jaws 90which frictionally engage with the rivet. The jaws 90 can be movedbetween a closed position in which they project radially inwards so asto grip a rivet 2, and an open position in which they are retractedwithin the tool nose. The distal tip of tool nose 50 also has a forcetransducer 92, which is connected to the control unit (not visible). Inthis embodiment the punch 48 is freely rotatable.

To mount a rivet 2 in the tool nose 50, with the jaws 90 open, the rivetis positioned with its pressure surface 28 abutting the distal surface58 of the punch 48 as described above. The jaws 90 are then closed, andeach clamps onto a circumferential portion of the filleted intersection26 of the rivet (shown in more detail in FIG. 1). With the clamp jaws 90acting on this sloped surface, the rivet 2 may tend to be cammed upwards(i.e. away from the workpiece 46), however this is prevented by thecontact between the pressure surface 28 of the rivet 2 and the distalsurface 58 of the punch 48. As shown in FIG. 6, with the rivet 2 mountedin the tool nose 50, its tip 6 projects from the distal end of the toolnose. In this embodiment, the rivet 2 projects from the tool nose 50 bya distance substantially the same as the thickness of the first layer 42of the workpiece 46.

The third embodiment also differs from the first embodiment in that thedie 52 is part of an ultrasonic horn 94. The ultrasonic horn 94 isconnectable to a source of ultrasonic energy (not visible), and acts tofocus ultrasonic energy produced by that source and direct it to theworkpiece 46 through the die 52. The ultrasonic horn 94 and the sourceof ultrasonic energy therefore co-operatively form an auxiliary heatingunit. In this embodiment, auxiliary heating in the form of ultrasonicenergy is applied to the workpiece 46 before and throughout the drivingof the rivet 2 into the workpiece 46. The control unit monitors theduration for which ultrasonic energy has been applied to the workpiece46, and begins to advance the punch 48 (and thus the tool nose 50 andrivet) once it has determined that the temperature of the workpiece 46will have been increased sufficiently by the ultrasonic energy. In thisembodiment, when the punch 48 (and thus the tool nose 50 and rivet 2) isadvanced, it is the rivet 2 rather than the nose piece 50 which contactsthe workpiece first. Indeed, since the rivet 2 projects from the toolnose 50 by a distance substantially the same as the thickness of thefirst layer 42 of the workpiece 46, the tool nose only contacts theworkpiece once the rivet has reached the second layer 44, as describedbelow.

As the rivet 2 is being advanced towards the workpiece 46, it is driven(by the tool nose 50 through the clamp jaws 90) to rotate at a speed of,for example, 4,000 RPM. Due to friction between the distal surface 58 ofthe punch 48 and the pressure surface 28 of the rivet 2, the rivetcauses the punch to rotate as well. Once the rivet contacts the top ofthe first layer 44, this layer has been heated sufficiently by theultrasonic energy applied through the die so that it requires relativelylittle friction stir heating. It is therefore unnecessary in thisembodiment for the axial movement of the rivet 2 to pause on the surfaceof the workpiece 46. Instead, the punch 48, rivet 2 and nose piece 50continue to advance and the rivet beings to penetrate the workpiece 46without delay. Although the carbon fibres of this layer are notsoftened, the matrix within which they are held is softened, allowingthe rivet to displace some fibres rather than cutting them. This reducesthe extent of fraying.

When the rivet 2 reaches the second layer 44, the control unit receivesfeedback from the force transducer 92 which indicates that the tool nose50 has contacted the workpiece. At that point, the control unit stopsrotation of the tool nose 50, and retracts the clamp jaws 90 so as todisengage the rivet from the tool nose. The axial advance of the punch48 continues during this time, but is slowed so that the clamp jaws 90are not damaged by movement of the punch 48 relative to the tool nose.This also ensures that the tool nose has come to a halt before it isurged against the workpiece with any significant force, which preventsthe tool nose from burrowing into the workpiece as described above.Since the rivet no longer rotating when it is driven into the secondlayer 46, the aluminium material is not heated to the point at which itwould be too soft for the rivet to be upset. The joint is then completedand the rivet upset.

Although in this embodiment the rivet does not pause on the surface ofthe workpiece due to the workpiece being softened by auxiliary heating,this is not intended to suggest that only when auxiliary heating is usedcan the rivet advance into the workpiece without pausing. In otherembodiments, for example, the rivet may have sufficient angular velocitywhen contacting the workpiece that friction stir softening occurs withsufficient speed that no pause in axial motion is required.

A method according to a fourth embodiment of the invention will now bedescribed with reference to FIG. 7. This embodiment is a method of solidriveting. However, unlike conventional solid riveting where mechanicalinterlock between the rivet and the workpiece is provided by upsettingthe rivet so as to produce enlarged portions on both sides of the rivet,in this embodiment the rivet 2 is not deformed at all. Indeed, the rivet2 of this embodiment is made out of ceramic material, which cannotusually be deformed to any significant extent before fracture takesplace. Further, in this embodiment the workpiece 46 is penetrateddirectly by the rivet 2, rather than the rivet being inserted into apre-formed hole.

Like the SPR rivets described above, the rivet 2 of this embodiment hasa substantially cylindrical shank 10, a tip 6 which has an annularcutting rim 20, and a head 8 with a frustoconical underside 24. In thiscase, however, the cutting rim 20 of the rivet 2 is provided around theperimeter of a concavity 100, rather than a bore. The rivet also hasdrive engagement features in the form of four substantially radialgrooves 102 (one of which is visible) in the head 8, arranged in asubstantially circumferential speed array at 90 degree intervals. Otherarrangements of grooves or ridges may be used.

The rivet 2 also has a surface irregularity in the radially outersurface 14 of the shank 10. In this case, the surface irregularity iselongate and takes the form of a groove 104 (which is an example of anopening). The groove 104 is arranged substantially circumferentially andruns round entire circumference of the rivet shank 10. It is thereforeannular in shape and provides the rivet shank 10 with a narrowed waist(that is to say a portion of reduced diameter).

The method of the fourth embodiment is a method of joining first andsecond layers 42, 44 of a workpiece 46, each of which is a part ofvehicle bodywork panels made of forming grade aluminium (although themethod could be used for other materials and/or products). The rivetingtool used in this embodiment has a tool nose 50 with a punch 48reciprocally received therein, as with SPR. In this case, however, thepunch 48 has a substantially annular array of four radial ridges 106(one of which is visible) for receipt within the radial grooves 102 ofthe rivet 2. In addition, the die 52 of this embodiment is a coiningdie. It has a flat surface 108 from which an annular lip 112 projectsupwards. The lip 112 surrounds the mouth of a bore 110 through the die52.

Unlike the previous embodiments, in this embodiment the rivet 2 isdriven into the workpiece 46 at a constant rotational velocity, e.g.6,000 RPM. Further, rather than actively controlling the linear motionof the punch 48, the control unit regulates the actuator (not visible)so that it provides a constant force, e.g. of 700N. This allows theaxial movement of the rivet 2 into the workpiece to be determined by thestate of the workpiece material in contact with the tip 6. The rivet 2is rotated as it approaches the workpiece 46. When the rivet 2 contactsthe workpiece 46, the aluminium material of the top layer 42 is at roomtemperature and is therefore insufficiently soft. The workpiece 46therefore provides axial resistance against the rivet 2, preventing itfrom moving into the workpiece. The rivet 2 therefore spins on the topsurface of the workpiece 46. Once the top region of the workpiece 46 hasundergone sufficient friction stir softening, the amount of axialresistance it applies to the rivet 2 drops. The downward axial force onthe rivet 2 begins to drive the rivet into the workpiece 46. As and whenthe rivet 2 contacts a portion of the workpiece 46 which isinsufficiently softened its axial motion will slow or stop, providingtime for the necessary frictional heating to take place.

As the rivet 2 travels into the workpiece 46, the portion of workpiecematerial beneath the concavity forms a slug 64 which is pushed downwardsalong with the rivet. When the rivet 2 has fully penetrated theworkpiece 46, the slug is sheared off between the rivet tip 6 and thelip 112 of the die 52, and falls through the bore 110 for disposal. Insome applications severing the slug 64 from the workpiece is desirableso as to provide a more flush finish on the underside of the workpiece(for instance for aesthetic or aerodynamic reasons). Removal of the slug64 at this stage also prevents it from detaching subsequently, forinstance after assembly of the finished product, at which point the slug64 could cause damage to other components of the product or could remainloose within the product and rattle whenever the product is moved.

Also as the rivet 2 penetrates the workpiece 46, the lower surface ofthe workpiece is pressed against the lip 112 of the die 52 withsufficient force that the lip produces an annular indentation in thebottom of the workpiece. This forces workpiece material from the lowerlayer 44 into the groove 104, which provides an interlock between thesecond workpiece layer and the rivet 2. With the head 8 of the rivetpreventing the first workpiece layer 43 from moving upwards relative tothe rivet, and the material in the groove preventing the lower layer 44from moving downwards relative to the rivet, the workpiece layers areheld together and the join is complete.

Since insertion of solid rivets such as the type described aboverequires plastic deformation of the workpiece 46 so as to allowpenetration by the rivet 2 and so as to provide mechanical interlockwith the rivet, the considerations in terms of hardness and ductility ofworkpiece layers described in the first to third embodiments is also ofrelevance to this technology. As such, altering the speed of rotationand/or axial insertion of a solid rivet 2 may provide one or more of thebenefits described in relation to SPR.

Although in this embodiment the workpiece 46 is urged against the lip112 of the die 52 by the force from the punch 48 (through the rivet) andtool nose 50, in other embodiments it may be so urged by only the punchor only the nose piece. Further, in other embodiments the die 52 may beurged upwards for at least part of the duration of rivet insertion,instead of or as well as the workpiece 46 being urged downwards by thetool nose 50 and/or punch 48.

FIG. 8 shows an alternative solid rivet suitable for use with the aboveembodiment (and suitable for use with modified versions of the aboveembodiment). The rivet 2 of FIG. 8 is similar to the rivet of FIG. 7,having a tip 6, a shank 10, and a head 8 with radial grooves 102.However, the rivet of FIG. 8 does not have a concavity (100 in FIG. 7).Instead, the tip 6 of the rivet 2 takes the form of a substantiallycircular surface positioned substantially normal to the rivet'slongitudinal axis 4. In addition, the groove 104 of the rivet 2 of FIG.8 is sufficiently wide that it extends along substantially the entireaxial length of the shank 10, and intersects the underside 24 of thehead 8. The greater volume of the groove 104 of the rivet 2 of FIG. 8may allow a greater volume of workpiece material to enter it incomparison to that of the rivet of FIG. 7, potentially increasing thestrength of the interlock between the workpiece layers 42, 44 and therivet 2.

Although as described above it may be beneficial for a rivet to have anannular cutting rim (20 in FIG. 7) so as to control the behaviour of theslug 64, some riveting methods may include an active slug removal step(such as grinding of the underside of the workpiece when the joint hasbeen formed), rendering such a feature unnecessary.

FIG. 9 shows a further rivet suitable for use with the invention. Thisrivet 2 is similar to the rivet of FIG. 7a , with the exception that ithas two surface irregularities on the radially outer surface 14 of itsshank 10, each in the form of a (substantially frustoconical) radialprotrusion 120. In this case, the protrusions 120 are positionedsubstantially circumferentially opposite to one another. The protrusions120 may provide mechanical interlock with the workpiece 64, for instanceby projecting into the lower workpiece layer 44. In addition, theprotrusions may provide a stirring action as the rivet 2 is insertedinto the workpiece 46 while rotating. This stirring action may act tomix softened material from the two workpiece layers 42, 44, producing anintermingled region and forming a friction stir spot weld. Such a weldmay provide supplementary strength to the joint, or may be used toprovide the entire connection between the workpiece layers 44, 46 (forinstance if the protrusions 120 of the rivet 2 of FIG. 9 project inbetween the two workpiece layers 42, 44).

Although the rivet 2 of FIG. 9 has two frustoconical protrusions 120positioned substantially circumferentially opposition one another, it isto be understood that other rivets may have a different number ofprotrusions, differently shaped protrusions, and/or a different numberof protrusions (for example 1, 3, 4 or more). The protrusions of otherrivets may be arranged in any suitable way, for instance they may beprovided in a substantially circumferential annular array (whetherevenly or unevenly spaced) and/or may be spaced at different axialpoints along the length of the rivet.

A further solid rivet 2 is shown in FIG. 10. Again, this rivet 2 issimilar to that described in relation to FIG. 7. In this case, the shank10 of this rivet 2 has surface irregularities in the form of twosubstantially diametrically opposite circular openings 122. Both theopenings 122 are provided by a substantially diametric through-bore 124.The opening 122 may receive workpiece material during insertion of therivet 2 (which may, for instance, be displaced by a coining die),providing the interlock between the rivet 2 and the workpiece layers 42,44.

Like the protrusions 120 of the rivet 2 of FIG. 9, other rivets may haveopenings of a different number, shape or location. For example, in onemodification of the rivet of FIG. 10, the openings may each be providedby a separate blind bore.

FIG. 11 shows an SPR rivet suitable for use with embodiments of theinvention. As with the solid rivets of FIGS. 7 to 10, the SPR rivet 2 ofFIG. 11 has surface irregularities in the radially outer surface 14 ofits shank. In this example, each surface irregularity is an elongateopening in the form of a groove 126. Each groove 126 is arrangedsubstantially longitudinally on the shank 10 of the rivet 2. As will beapparent from FIG. 11, the grooves 126 are of different longitudinallengths, and are arranged substantially evenly in a substantiallycircumferential array around the rivet shank 10. The grooves 126 mayimprove (or provide, in other embodiments where the rivet is not upset)the required interlock between the rivet 2 and the workpiece, and/or mayprovide the stirring action described above.

A further solid rivet 2 suitable for use with embodiments of theinvention is shown in FIG. 12. This rivet 2 is similar to that describedin relation to FIG. 7, except that the surface irregularity in theradially outer surface 14 of the shank 10 of this rivet 2 takes the formof an elongate groove 128 which is helical in shape. This groove 128 mayprovide the stirring action and/or mechanical interlock described above.The helical nature of the groove 128 may also act as a screw threadduring insertion of the rivet 2 into a workpiece while the rivetrotates. For example, during insertion of the rivet 2, workpiecematerial may be accommodated in the portion of the groove 128 nearestthe tip 6 of the rivet. This workpiece material may cause the rivet 2 tobe urged further into the workpiece as it continues to rotate, in amanner akin to the driving of a screw. Alternatively, helical formationssuch as the groove 128 may instead (or as well) act to provide astirring action or provide mechanical interlock as discussed above. Forexample, the rivet 2 of FIG. 12 may be driven into a workpiece whilerotating in the opposite direction to that which would allow the groove128 to act as a screw thread).

Although in this example the rivet 2 has a single helical groove 128,other rivets may have two or more intertwined helical grooves (forming,for example, a double helix). Further, although in this example thegroove 128 completes more than one full revolution of the rivet shank10, other embodiments may have one or more grooves which are in theshape of a portion of a helix but that do not complete an entirerevolution around the rivet shank. Such grooves may be considered to bein the shape of a helical arc.

FIG. 13 shows a further SPR rivet for use with the invention (FIG. 13a), components of a riveting tool used to insert the rivet 2 into aworkpiece 46 (FIG. 13b ), and a joint formed using this rivet andriveting tool (FIG. 13c ). The rivet 2 of FIG. 13 is similar to that ofFIG. 1, therefore only the differences will be described here. Thisrivet 2, like the rivets of FIGS. 7 to 12, has surface irregularities.In this case, the rivet 2 has four surface irregularities 140 a-140 d.Each surface irregularity 140 a-140 d takes the form of a helical rib(i.e. an elongate protrusion of helical shape). The four helical ribs140 a-140 d are intertwined with one another and each end circle theshank 10 of the rivet 2, making approximately 1¼ revolutions of therivet shank. The ribs 140 a-140 d may provide any of the advantagesdescribed in relation to the groove of the rivet of FIG. 12.

The rivet 2 of FIG. 13 also differs from that of FIG. 1 in that the head8 has engagement features in the form of four substantially radialgrooves 142 provided in the pressure surface 28. The grooves 142 arearranged in a substantially circumferential array at approximately 90°intervals, and each groove 142 tapers in depth towards its radiallydistal end. The grooves 142 co-operatively form a cross-shapedindentation in the pressure surface 28 akin to a Philips drive socket.In addition, the rivet 2 of FIG. 13 differs from that of FIG. 1 in thatits internal taper surface 18 is arcuate, providing the bore 12 with amouth that is trumpet-shaped. In other words, while the taper surface ofthe rivet of FIG. 1 is a chamfer, the taper surface 18 of the rivet 2 ofFIG. 13 is a fillet.

As shown in FIG. 13b , the grooves 142 in the pressure surface 128 ofthe rivet head 8 are each engaged by complimentarily shaped and spacedridges 144 in the distal surface 58 of the punch 48 (the ridges 144co-operatively forming a cross-shaped projection akin to a Phillipsdrive bit). FIG. 13b also illustrates an SPR die 52 of slightlydifferent geometry than that of FIGS. 4 to 6. Whilst the dies shown inthese Figures have a pip 54 which is slightly recessed behind thesupport surface 57 of the die, in this case the pip 54 projects abovethat surface. Furthermore the cavity 56 in the die of FIG. 13b is ofreduced depth in comparison to that of the die of FIGS. 4 to 6.

FIG. 13c shows a joint produced using this rivet, and shows thatinterlock between the rivet 2 and workpiece 46 is not only provided byflaring of the rivet shank 10, but also by workpiece material beingreceived between the ribs 140 a-140 c.

FIG. 14 shows another rivet 2 suitable for use with embodiments of theinvention, in this case a solid rivet. This rivet 2 is similar to therivet of FIG. 7, apart from the fact that it has two surfaceirregularities, each in the form of an elongate groove 144. The grooves144 are curved along their length, but are not arranged helically aroundthe rivet shank 10. Instead, each groove 144 follows an asymptoticcourse.

Whilst the rivets of FIGS. 7 to 14 have surface irregularities on theouter surfaces of their shanks, where a rivet has a bore defining aradially inner surface, the rivet may have one or more surfaceirregularities provided in this radially inner surface. FIG. 15 shows amodification of the rivet of FIG. 13 which includes such internalsurface irregularities. More specifically, the rivet 2 of FIG. 15 hashelically arranged ribs 140 a, 140 b, as also found in the rivet of FIG.13. However, in this case, the ribs 140 a, 140 b are provided on theradially inner surface 16 of the shank 10. These helical ribs 140 a, 140b may provide the functionality described in relation to the ribs ofFIG. 13. The rivet 2 of FIG. 15 also has circular openings 122. Theseopenings 122 each take the form of a substantially radial bore whichpasses through the radially inner and outer surfaces 16, 14 of the shank10. Since the openings 122 intersect both the radially inner and outersurfaces 16, 14, they may be considered to be positioned both internallyand externally (i.e. positioned on the inner surface and on the outersurface). In an alternative arrangement, each opening 122 may bereplaced by a pair of blind bores, one in the radially outer surface 14and one in the radially inner surface 16 (these blind bores notnecessarily being aligned with one another).

Internal surface irregularities may allow a slug of workpiece materialintroduced into the bore 12 during insertion of a rivet 2 to bepositively retained therein, minimising the possibility of slugs workingloose and affecting the performance of a finished product (as describedabove). For example, in the rivet 2 of FIG. 15 workpiece material fromthe slug may fill portions of one or more of the openings 122 and/orpart of the space between the internal helical ribs 140 a, 140 b. Thiswould provide an interlock between the rivet 2 and the slug, so as toretain the slug.

FIG. 16 shows the tip 6 and shank 10 of another SPR rivet suitable foruse with embodiments of the invention. This rivet has helical ribs 140a, 140 c of the type described in relation to FIG. 13, and also hasinternal circumferential grooves 104 a, 104 b. The helical ribs 140a-140 c may function as described in relation to FIG. 13, and thecircumferential internal grooves 104 a, 104 b may act to retain a slugof workpiece material as described above. The bore 12 of the rivet ofFIG. 16 also has a surface irregularity in the form of a shoulder 146,which may also acts to provide interlock between the rivet and a slug ofworkpiece material if workpiece material in the bore 12 is deformed soas to fill the space behind the shoulder.

In some applications it may be desirable for rivets to have acircumferentially discontinuous tip. A circumferentially discontinuoustip may be considered to be a tip which, in a plane normal to thelongitudinal axis of the rivet, is not uniform about its circumference.An exemplary rivet with a circumferentially discontinuous tip is shownin FIG. 17. This rivet 2 is similar to that of FIG. 1, except that ithas a substantially circumferential array of longitudinal slots 150. Theslots 150 extend to the tip 6 of the rivet 2 and along around 70% of theaxial length of the shank 10. In this case, the slots 150 intersect boththe radially outer and radially inner surfaces 14, 16 of the shank 10.In other words, they extend radially through the entire thickness of thecylindrical portion of the shank 10.

The tip being circumferentially discontinuous may be beneficial inallowing the rivet tips to cut or drill its way into a workpiece ratherthan merely displacing workpiece material. The circumferentiallydiscontinuous tip 6 being provided by longitudinal slots 150 whichextend into a significant portion of the shank 10, may also bebeneficial in that the shank is weakened circumferentially. This mayreduce the amount of force which must be provided by the plastic flow ofworkpiece material so as to upset the rivet.

FIG. 18 shows another SPR rivet 2 suitable for use with the invention.As with the rivet of FIG. 17, the rivet 2 of FIG. 18 has acircumferentially discontinuous tip 6 provided by slots 150. In thiscase, however, the slots 150 extend longitudinally a short distance intothe shank 10 (e.g. less than one quarter of the length of the shank).This maintains the strength of the shank, which may allow it to pierceharder workpiece materials without flaring prematurely. The rivet 2 ofFIG. 18 also has drive engagement features 152 in the form of foursubstantially radial ridges arranged in 90° increments. These ridges 152are configured to be received in corresponding substantially radialgrooves in a punch.

FIG. 19 shows another SPR rivet 2 with a circumferentially discontinuoustip 6. In this case, the circumferentially discontinuous tip is providedby a substantially circumferential array of substantially evenly-spacedcutting teeth 154. This rivet 2 has an internal taper surface 18 and anexternal taper surface 30, giving the teeth a substantially pyramidalprofile. The teeth 154 of this rivet coming to a sharp point in thismanner may provide a particularly aggressive action. The teeth may,however be less resistant to deformation or wear of the tip 6 duringinsertion into a particularly hard material, in comparison to the rivetsof FIGS. 17 and 18.

FIG. 20 shows another SPR rivet 2 with cutting teeth 154. The teeth 154of this rivet have a saw-tooth profile, which may allow theaggressiveness of the cutting action to be altered according to thedirection in which the rivet 2 is rotated. More particularly, the rivetwould provide a more aggressive cutting action if the rivet were rotatedclockwise (when viewed from the top of the head 8) than if it wererotated anticlockwise. In the case of the rivet 2 of FIG. 20, the teethterminate in a substantially radial linear edge, rather than a point.This may make the teeth 154 more resistant to wear or deformation duringinsertion of the rivet, in comparison to the rivet of FIG. 19.

Another self-piercing rivet suitable for use in the invention is shownin FIG. 21. The circumferentially discontinuous tip 6 of this rivet 2 isprovided by the orientation of the cutting rim 20. More particularly,whereas in the previous rivet designs the tip defined a plane which wassubstantially perpendicular to the longitudinal axis of that rivet, inthis case the rim 20 lies in a plane 160 positioned at an angle 162 ofaround 80° to the rivet longitudinal axis 4. The rivet 2 of FIG. 21 alsohas a different type of drive engagement feature than the rivets ofFIGS. 17 to 20. In this case, the head 8 of the rivet 2 has a socket 164of a substantially hexagonal cross-section which is configured toreceive a complementarily-shaped hexagonal projection of a punch.

Another rivet 2 with a circumferentially discontinuous tip 6 isillustrated in FIG. 22. In this case, the rivet 2 is a blind rivet, witha main body 170 configured to be deformed by pulling on a mandrel 172 ina manner with which the skilled person will be familiar. In this case,the circumferentially discontinuous tip 6 of the rivet 2 is provided onthe bulb 174 of the mandrel 172, and the shank 10 (provided by the mainbody 170) is spaced therefrom by the proximal portion 176 of the bulb174 (the portion of the mandrel 172 which upsets the main body 170). Thetip 6 takes the form of a (external) taper surface 30 which tapers to apoint 173 that (in this embodiment) is intersected by the longitudinalaxis (not shown) of the rivet 2. The taper surface of this rivet 2 isfaceted. More specifically, the taper surface 30 has four facets 175arranged substantially in the shape of the sides of a pyramid. The rivet2 of FIG. 22 also has a drive engagement flat 178 provided on themandrel, for engagement by a grub screw of a rotary drive component. Thehead 8 of the rivet 2 is provided by the main body 170, and has radialridges 152 of the type described in relation to FIG. 18.

FIG. 23 shows a further modification of the rivet of FIG. 1. In thiscase, the bore 12 extends throughout the entire axial length of therivet 2. This may be beneficial in reducing the circumferential strengthof the rivet (by removing any mechanical support provided by the headbeyond the end of the bore), thereby allowing the shank to be upset moreeasily. A fully tubular rivet may also be beneficial in that spacewithin the bore is less restricted. If the space in the bore was toosmall, when the bore was full additional workpiece material would bedisplaced downwards along with the rivet (rather than into the bore),inhibiting penetration.

FIG. 24 illustrates a modification of the rivet of FIG. 23. In thiscase, the bore 12 includes a drive engagement portion 190, which is aportion of the bore that is non-circular in cross section.

In this case the drive engagement bore portion 190 is positioned at theaxial end of the bore 12 (and thus of the rivet 2) opposite to the tip6, however in other cases it may be differently positioned. In thisexample, the drive engagement bore portion 190 is substantiallyoctagonal in cross-sectional shape. The drive engagement bore portion190 is configured to receive a driving bit in the form of acomplementarily-shaped drive engagement projection 192 provided on thedistal surface 58 of a punch 48. This is shown in FIG. 24B. Where isdesirable for workpiece material to enter the bore 12 alongsubstantially its entire length, for instance to avoid the resistance toinsertion due to the bore being full (as described above), or so thatthe top of the slug in the finished joint lies substantially flush withthe pressure surface 28 of the rivet, the drive engagement projection192 of the punch 8 may be retractable. This may allow the driveengagement projection 192 to be (at least partially) withdrawn from thedrive engagement bore portion 190 while the punch 48 is still in contactwith the rivet 2. More particularly, the drive engagement projection 192may be retractable to a position in which it is flush with the distalsurface 58 of the punch 48, as illustrated in FIG. 24C, or may beretractable to a position in which it still projects from the distalsurface 58 but does so to a reduced extent.

It is to be noted that although in FIG. 24 the drive engagement boreportion 190 is of larger diameter than the remainder or the bore 12, inother embodiments the drive engagement bore portion may be of smallerdiameter, or of equal diameter but of different shape. Further, in otherarrangements the drive engagement bore portion 190 may extendsubstantially the entire length of the bore.

Another fully tubular rivet (i.e. a rivet with a bore extending alongits entire axial length) is shown in FIG. 25. This rivet 2 issubstantially symmetrical about its longitudinal axis 4 (that is to saythat it is symmetrical in a plane which is normal to its longitudinalaxis). Either axial end of the rivet 2 may be considered to be the tip6, the rim 20 of that end being used to pierce a workpiece and the rim20 of the other end acting as the contact surface 28 of the rivet. Therivet 2 being substantially symmetrical along its longitudinal axis 4,allowing either end of the rivet to function as the tip 6, may bebeneficial in allowing the rivet to be fed to the tool nose of ariveting tool in either axial orientation. In contrast, where a rivethas a single end which is suitable for piercing a workpiece, a rivetingtool may have to include an orientation mechanism to ensure that therivet is mounted within the tool nose with the rivet tip pointingdownwards. Such an orientation mechanism may add complexity and/or bulkto the riveting tool.

FIG. 26 illustrates a method of inserting the rivet 2 of FIG. 25 into aworkpiece 46, with stages of this method shown as FIGS. 26a-26c . Likethe riveting tool of the above embodiments, in this example the tool hasa punch 48, a tool nose 50, and a die 52 with a pip 54 and a cavity 56.

In this case, however, the tool also has a pair of counterposed supportmembers 200. Each support member has an arcurate indentation 202 ofcomplimentary shape to the shank 10 of the rivet, and is arranged toreceive a portion of the shank. The tool of FIG. 26 also differs fromthose of the above embodiments in that the punch 48 has a profiled tip.More specifically, the distal surface 58 of the punch 48 has a domedprojection 204 protruding therefrom. Furthermore, although in theprevious embodiments the punch 48 was substantially the same diameter asthe rivet, in this case the punch 48 is significantly larger in diameterthan the rivets 2.

To insert a rivet 2 into a workpiece 46, the rivet 2 is mounted betweenthe support members 200. With the die 52 positioned against the bottomsurface of the workpiece 46, and the support members 200 resting on atop surface, the rivet 2 is advanced and rotated by the punch 48 (whichacts as a rotary drive component in frictional engagement with therivet).

While the rivet is being driven into the first layer 42 of theworkpiece, it is in engagement with the support members 200 so the shank10 is radially supported and prevented from deforming outwards. When therivet 2 reaches the second workpiece layer 44, however, the supportmembers 200 move away from each other, disengaging their indentations202 from the shank 10 of the rivet 2. This provides space for the punch48 to pass between the support members 200. Further, it provides roomfor the upper end of the rivet 2 (i.e. the axial end of the rivetopposition the tip 6) to be flared outwards under action of theprojection 204. This provides interlock between the top end of the rivetand the top end of the rivet 2 and the upper layer 42 of the workpiece46. Simultaneously, plastic flow of workpiece material in the bottomlayer 44 flares the tip 6 of the rivet as described previously. This isshown in FIG. 26 c.

Although in this case two support members 200, positioned opposite toone another, are used to prevent premature layering of the rivet 2, andother embodiments may support the rivet in other ways. For example,other embodiments may use a different number of support members 200 (forexample 3, 4 or more support members), and the support members may bearranged in any other suitable configuration. For instance, in anotherembodiment four support members may be used, the support members beingpositioned at a different circumferential locations around the rivet. Inanother modification of the embodiment of FIG. 26, the shank of therivet may not be supported by support members at all. For example, thegeometry of the rivet may be such that without support the top end ofthe rivet is deformed to some extent, but the rivet is still able topenetrate the workpiece 46 effectively. Where support members are used,these may be disengaged from the rivet at any other suitable time. Thesupport members may also be of different shape. For instance, they mayhave a flat surface which contacts the rivet rather than having anindentation, or they may have an indentation of a different form.Support members, where present, may or may not be substantiallyidentical to one another.

In other arrangements in which a riveting tool has a punch with aprofiled tip for deforming the end of a rivet that is substantiallysymmetrical along its longitudinal axis, the profiled tip may include aconical or pyramidal (or frustoconical or frustopyramidal) projectionprotruding from a distal surface of the punch, rather than a domedprojection, or the entire distal end of the punch may take such a shape.Alternatively, the punch tip may be profiled in any other suitablefashion.

During insertion of a rivet into a workpiece, some workpiece materialcan be forced upwards, above the original top surface of the workpiece.In addition, where a rivet penetrates a workpiece through a cutting ordrilling action, it can generate swarf. For similar reasons as theretention of workpiece slugs, as described above, in some applicationsit may be desirable to positively retain this material rather thanprovide the opportunity for it to detach at a later point. Rivets maytherefore be provided with one or more cavities in the underside oftheir heads, or in a portion of their shanks adjacent thereto, so as toretain this material. FIGS. 27-29 illustrate three such rivets, which inthis case are SPR rivets.

The rivet 2 of FIG. 27 has an annular cavity 210 provided in theunderside 24 of its head 8, the rivet 2 of FIG. 28 has an annular cavity201 positioned in the portion of the shank 10 adjacent to the underside24 of the head 8. FIG. 29 shows a rivet 29 which has an annular cavitypositioned at the junction between the underside 24 of the head 8 andthe portion of the shank 10 adjacent thereto. The cavity 210 of FIG. 29may instead be considered to be two conjoined cavities, one in the head8 and one in the shank 10.

The cavities 210 of each of the rivets 2 are positioned substantiallycircumferentially. However, in other embodiments they may be positionedin any other suitable way. Further, other rivets may include cavitiesfor retaining workpiece material which are not annular. For instance,one such rivet may have a circumferential array of three separatecavities. The a rivet such as those shown in FIGS. 27 and 28 may bedriven into a workpiece so that the underside 24 of the rivet contactsthe top surface of the workpiece, which may provide more completeretention of workpiece material (since no material is displaced by thehead). Alternatively, it may be driven into the workpiece so that itspressure surface 28 is substantially flush with the workpiece, which mayproduce a smoother surface in the region of the joint.

In some applications, during insertion of a rivet the top layer orlayers of a workpiece can leave a hole of larger diameter than the shankof a conventional rivet. For instance, where the top layers of aworkpiece are significantly softer than one of the other layers, it maybecome over-softened (for instance it may melt) and be forced radiallyoutwards away from the rivet by centrifugal force. This may provide agap between the rivet and the top layers of the workpiece, which may beunaesthetic and/or provide a route for ingress of moisture. It maytherefore be desirable to utilize a rivet which has a portion of itsshank at its head end that is radially larger than the remainder of theshank. FIGS. 30 and 31 illustrate two such rivets. The rivet 2 of FIG.30 has a stepped shank 10 comprising a first shank portion 10 a adjacentto the tip 6, and a second shank portion 10 b adjacent to the head 8.The diameter of the second shank portion 10 b is larger than that of thefirst shank portion 10 a, so that if the top layer or layers of aworkpiece expands away from the rivet when penetrated by the first shankportion 10 a, the enlarged hole produced can be filled by the secondshank portion 10 b. The rivet 2 of FIG. 31 has a shank 10 whichcontinuously tapers from the head 8 to the tip 6. In a similar fashionto the rivet of FIG. 30, the portion of the shank 10 at the head end ofthe rivet can fill a hole in the top workpiece layer (s) which is largerin diameter than the portion of the shank at the tip end.

It will be appreciated that numerous modifications to the abovedescribed embodiments may be made without departing from the scope ofthe invention as defined by the appended claims. For instance, whilstclamping of a workpiece between a die and a tool nose has only beendescribed in relation to a particular embodiment, it may equally be usedin any other suitable embodiment. In addition, whilst auxiliary heatingof a workpiece has been described as being supplementary to frictionstir heating, in other embodiments a rivet may be driven into aworkpiece at insufficient rotational velocity to bring about anysignificant friction stir heating, at which point substantially all theincrease in temperature of a workpiece necessary to soften the workpiecematerial may be brought about by auxiliary heating. In any embodiment ofthe invention, auxiliary heating (where present) may take any suitableform, such as the application of ultrasonic energy, the use of laser,the use of electromagnetic induction, or the use of a radiative orconvective heating element such as a heating coil or a gas burner.Further, auxiliary heating of more than one type may be utilised (eitherat different times or simultaneously) in a single embodiment.

In any embodiment of the invention which utilises a die and a punchand/or tool nose, the die and the punch and/or tool nose may be mountedopposite one another on a force reaction frame such as a C-frame.

Although some embodiments of the invention relate to SPR riveting andothers to solid riveting, it is to be understood that one or morefeatures described in relation to SPR may also be present in relation tosolid riveting, and vice versa. Similarly, the insertion of a blindrivet may also utilise friction stir softening with an alteration in therotational speed and/or axial speed of insertion of the rivet, thereforeone or more of the aspects of the invention (and any appropriateoptional features) may be embodied by a blind riveting process. In thiscase, the method would further comprise moving the mandrel of a blindrivet inserted into a workpiece relative to the main body of that blindrivet to upset the rivet. The anatomy of a blind rivet, and theupsetting of that blind rivet (after insertion into a workpiece using amethod according to the invention) would be readily apparent to theskilled person.

Whilst specific types of auxiliary heating, namely the use of a laserand the use of ultrasonic energy, have been described in relation toparticular embodiments, these types of auxiliary heating (or any other)may be utilised in relation to any embodiment of any aspect of theinvention.

Although embodiments described above relate to the manufacture of amotor vehicle, embodiments of the invention may be used in themanufacture of any other suitable products. Such products includeindustrial machinery, air conditioning systems, road signs, mechanicalsupport structures, water tanks and grain silos.

In any embodiment of the invention where the rivet has a bore, the boremay be a blind bore or a through bore. It may be substantially circularin cross section, or may have any other suitable cross sectional shape.The bore may be parallel with longitudinal axis of the rivet, forinstance it may be collinear therewith. The bore may or may not changein diameter and/or in cross sectional shape along its length.

Although the second and third embodiments of the invention (i.e. thosein which the tool nose functions as the rotary drive component) describethe tool nose engaging the rivet frictionally and about itscircumferential periphery, in other embodiments the tool nose may engagea different portion of the rivet and/or may not engage the rivet throughfriction. Similarly, the rivet may engaged about its peripherynon-frictionally, and/or by a component of the riveting tool other thanthe tool nose. By way of an illustration, the rivet may have anon-circular head (for instance an octagonal head), and be receivedwithin a complimentarily-shaped bore (for instance an octagonal bore, ora square bore) in the nose piece. In this case, the rivet would beengaged by the tool nose non-frictionally. As another illustration, therivet may have a head which is non-circular in cross section and whichis received within a complimentarily-shaped socket in the punch (forinstance the rivet head may be pentagonal and received in a pentagonalsocket in the punch). In this case, the rivet would be engaged about itscircumferential periphery, but would be engaged non-frictionally andwould not be engaged by the tool nose.

It is to be understood that reference herein to a metal is intended toinclude alloys of that metal. Although in the second embodiment thepressure surface of the riveting tool (provided on the clamping washer,in that example) is freely rotatable relative to the tool nose due to itbeing mounted thereto by bearings, in other embodiments the pressuresurface may be coupled to the tool nose in an alternative manner inwhich the relative rotation between the pressure surface and the toolnose is restricted. This may be advantageous in that the pressuresurface may be urged to rotate on the surface of the workpiece to someextent, thereby generating frictional heat.

Whilst support members have been described in relation to supporting arivet which is substantially symmetrical along its longitudinal axis,they may be utilised in other embodiments. For instance, they maysupport a rivet which is fully tubular but which has a head portion, soas to compensate for the reduced strength described above (or in caseswhere the rivet is deformed at the head end by a profiled punch to as toincrease the radius of the head). Further, they may be used to supportany other type of rivet. For instance, a relatively soft rivet may beused (for example for the sake of corrosion resistance), the reducedcolumn strength of that rivet being compensated for by support from thesupport members.

In relation to solid riveting, although the solid rivets described aboveeach comprise a radially enlarged head, in other embodiments a solidrivet may not comprise such a head. In this case, the mechanicalinterlock between the rivet and the workpiece layers may be broughtabout entirely by deformation of workpiece material into or aroundsurface irregularities (for instance the rivet may have two grooves, oneof which receiving material from the top workpiece layer and the otherreceiving material from the bottom workpiece layer).

Although dies of particular formats have been described in the aboveembodiments, other embodiments of the invention may utilise a die of adifferent form. For example, an embodiment may utilise a die whichcomprises a flat surface, a die which has a recess with no pip, a diewhich has a pip with no recess, a die with a convex support service or adie of any other suitable shape. Further, in some embodiments in theinvention (for instance those which utilise a blind rivet) no die may beused.

In some embodiments, a die of a particular shape may be utilised in anumber of different riveting processes (for instance the joining ofworkpieces of different thicknesses) comprising different materials, orutilizing different rivets). By way of an illustration, rivets of 10 mmaxial length may be used in combination with the same die to joinworkpieces of total thickness 8 mm or 12 mm. In the case of a 12 mmthickness workpiece, the pip of the die may still be able to direct theplastic flow of workpiece material sufficiently to fully upset therivet, even though the workpiece material may not enter the cavity ofthe die to its full depth the rivet itself would not enter the cavity.In the case of an 8 mm thick workpiece the workpiece material may enterthe cavity to an increased depth, allowing the tip of the rivet to enterthe cavity while remaining fully encapsulated.

Although SPR has been described in relation to the rivet being upsetinto the bottom layer of a workpiece, thereby being encapsulated withinthe workpiece, in another form of riveting the rivet may fully penetratethe workpiece. For example, the rivet may fully penetrate the workpiecewithout being deformed to any significant extent, before being upset bya die so as to flare the distal portion of the shank underneath thelower surface of the workpiece. This may be useful when rivetingworkpieces where the bottom layer is of insufficient ductility for therivet to be upset therein.

Whilst the above embodiments describe examples of ways in which ariveting process may be monitored, any embodiment may utilise anysuitable arrangement for collecting information. For example, therotational and/or linear position or speed of the rivet (or a componentof the riveting tool) may be monitored by a control unit using one ormore positional encoders such as optical (e.g. laser), magnetic,inductive, capacitive or eddy current encoders. Similarly, althoughtemperature sensors and force (or torque) transducers have each beendescribed above in relation to a particular location and in connectionwith numerous other features, one or more such transducers (or atransducer of any other type) may be included in any suitable position(such as in the nose of the tool, the punch, or the die) and may be usedin any suitable embodiment of any aspect of the invention. Similarly,the duration of a particular action (such as the time for which a rivethas been moved axially or rotationally, or the duration for whichauxiliary heating has been applied) may be monitored in any embodimentof the invention and used to determine when a subsequent action shouldbe initiated. Alternatively, embodiments of the invention may notcollect feedback of any sort. For instance the rivet may be insertedusing a riveting tool the actions of which are controlled entirely by anoperator.

Although the above embodiments describe the rivet being moved axiallyinto a stationary workpiece, in other embodiments the workpiece may bemoved along the longitudinal axis of the rivet into a stationary rivet,or both the rivet and workpiece may be moved together. Accordingly,reference herein to the speed of a rivet (whether rotational or axial)should be interpreted as referring to the speed of the rivet relative tothe workpiece.

Whilst the above embodiments describe particular combinations of numbersof workpiece layers, workpiece layer materials, rivet materials, axialspeeds (and alterations thereof) and rotational speeds (and alterationsthereof), this is not intended to imply that these selections are onlysuitable for use in the combinations described. Any suitable combinationof numbers of workpiece layers, workpiece layer materials, rivetmaterials, axial speeds (and alterations thereof) and rotational speeds(and alterations thereof) are contemplated.

In any embodiment of the invention in which the axial speed of the rivetis adjusted, the rotational speed of the rivet may be constantthroughout insertion. Similarly, where the rotational speed of the rivetis adjusted, the axial speed of the rivet may be constant throughoutinsertion.

Although particular embodiments described above utilise a hydraulicmotor or an electric motor to rotate the rivet, any embodiment of theinvention may use any suitable arrangement to rotate the rivet. Forexample, an embodiment may utilise an electric motor (such as aninduction motor, a synchronous motor or a DC motor) a hydraulic motor ora pneumatic motor. Similarly, although the above embodiments describeuse of a hydraulic cylinder or an electric linear actuator to drive therivet axially, any embodiment of the invention may utilise any suitableform of actuator, such as a solenoid, hydraulic cylinder, pneumaticcylinder, or electric linear actuator.

In an embodiment of the invention where the rivet includes one or moresurface irregularities, these may be sized and/or positioned tocorrespond with the location of one or more specific workpiece layers.

Although the description above refers to a longitudinal slotted shankand an array of teeth as being alternatives, it is to be understood thata rivet may incorporate both of these features. For example, a rivet mayhave longitudinal slots which provide circumferential portionstherebetween, each circumferential portion having one or more teeth. Asanother alternative, longitudinal slots may provide circumferentialportions each of which are tapered towards their distal tip so as toprovide an integral tooth.

Where a rivet has substantially longitudinal slots in a substantiallytubular portion of its shank, the slots may or may not run the fullaxial length of the tubular portion, and where the slots do run the fullaxial length of the tubular portion they may or may not run the fullaxial length of the entire shank. Although in the above example theslots are of uniform longitudinal width and uniform radial depth, inother embodiments this may not be the case. For instance, thecircumferential width of the slots may increase or decrease (uniformlyor non-uniformly) towards the tip. Alternatively or in addition, theradial depth of the slots may increase or decrease (uniformly ornon-uniformly) towards the tip. In the described example the slots aresubstantially identical and are arranged substantially uniformly,however in other embodiments the slots may differ from one another (forexample in longitudinal length, circumferential width or radial depth)and/or be differently arranged.

In situations where a slug of workpiece material is at risk of detachingfrom a finished joint after assembly of a finished product, a rivet maybe provide with a quantity of adhesive (for instance in the bore of anSPR rivet or the concavity of a solid rivet) which may adhere the slugto the rivet and reduce the likelihood of the slug detaching. This maybe used in combination with one or more of the above described ways inwhich the slug can be retained, or may be used in isolation.

Whilst workpiece layers of a range of materials have been described, theselected range of materials should be considered to be merely examples.In an embodiment of the invention a workpiece may contain one or morelayers of any suitable material, such as magnesium, aircraft gradealuminium, ultra-high strength steel, titanium or metal matrixcomposite. Alternatively or in addition, the workpiece may comprise oneor more layers of carbon fibre composite, glass fibre composite,polymer, a metal such as standard grade steel (stainless ornon-stainless) or forming grade aluminium, an organic material such asleather, or any other suitable material.

For the sake of simplicity, changes to the motion of the rivet, anddeformation of the rivet (where applicable) have been described asoccurring layer-by-layer in stepwise fashion. However, it is to beunderstood that in reality deformation of the rivet and/or alterationsin its rotary and/or linear speed may not occur in this manner. As anexample, events described as taking place suddenly when the rivetcontacts a workpiece layer of harder material may in fact take place ina gradual fashion as the rivet contacts workpiece material whichgradually is colder (and therefore harder), whether or not this coldermaterial is part of a newly-contacted workpiece layer. Accordingly, themotion of the rivet may be controlled according to the forces itexperiences (such as resistive forces) rather than its position relativeto the layers of a workpiece.

Although the different aspects of the invention have been describedindividually or particular combinations, it is to be understood thateach aspect of the invention may be utilised individually or incombination with any other aspect or aspects of the invention whereappropriate. For example, although the rivet being engaged about itscircumferential periphery (the seventh aspect of the invention) has beendescribed in combination with the application of auxiliary heating (theeleventh aspect of the invention), a method according to the inventionmay utilise only one of these concepts. As another example, a rivet mayhave a bore that extends along its entire axial length (in accordancewith the eighth aspect of the invention) and a cavity provided in theunderside of its head (in accordance with the ninth aspect of theinvention), and be driven to rotate by the nose of a tool (in accordancewith the eighth aspect of the invention). Indeed, a rivet with both anenlarged shank portion at the head and a cavity to receive workpiecematerial may be beneficial in that the enlarged shank portion can bedeliberately oversized. This may ensure that the rivet can fill a holein the top portion of the workpiece of any size which may reasonably beexpected, with the cavity receiving workpiece material displaced by theenlarged shank portion in the event that the hole in the top of theworkpiece is of smaller diameter.

Further, although specific features or arrangements have been describedin relation to one aspect of the invention, some may equally beconsidered as relating to a different aspect of the invention. Forinstance, the rivet of FIG. 14 has been described in terms of thegrooves being surface irregularities (i.e. it has been described inrelation to the first aspect of the invention). However, these groovesalso provide the rivet with a circumferentially discontinuous tip(albeit one which is not circumferentially discontinuous across itsentire radial extent), therefore this rivet may also be considered torelate to the second aspect of the invention. As another example, therivet of FIG. 17 has been described in terms of the slots providing acircumferentially discontinuous tip (i.e. it has been described inrelation to the second aspect of the invention), the slots alsoconstitute surface irregularities in the form of openings therefore thisrivet may also be considered to relate to the first aspect of theinvention.

Optional and/or preferred features as set out herein may be used eitherindividually or in combination with each other where appropriate andparticularly in the combinations as set out in the accompanying claims.The optional and/or preferred features for each aspect of the inventionare also applicable to any other aspects of the invention whereappropriate.

1. A method of inserting a rivet into a workpiece, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: the rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece; the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete; one axial end of the rivet has a tip for piercing the workpiece, and the rivet has a substantially cylindrical shank extending longitudinally from the tip; and the shank has one or more surface irregularities.
 2. A method according to claim 1 wherein one or more of said surface irregularities are provided on a radially outer shank surface.
 3. A method according to claim 1 wherein the rivet has a bore which runs through the tip and through at least a portion of the shank, thereby providing a radially inner shank surface, and one or more of said surface irregularities are provided on the radially inner shank surface.
 4. A method according to claim 1 wherein one or more of said surface irregularities are elongate in shape
 5. A method according to claim 4 wherein one or more of said elongate surface irregularities are aligned substantially longitudinally.
 6. A method according to claim 4 wherein one or more of said elongate surface irregularities are aligned substantially circumferentially.
 7. A method according to claim 4 wherein one or more of said elongate surface irregularities are each substantially in the shape of a helical arc.
 8. A method according to claim 1 wherein one or more of said surface irregularities each take the form of a projection.
 9. A method according to claim 1 wherein one or more of said surface irregularities each take the form of an opening.
 10. (canceled)
 11. A method of inserting a rivet into a workpiece, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: the rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece; the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete; and one axial end of the rivet has a circumferentially discontinuous tip for piercing the workpiece, and the rivet has a substantially cylindrical shank which extends longitudinally from the tip and provides a radially outer shank surface;
 12. A method according to claim 11 wherein the circumferentially discontinuous tip comprises a plurality of teeth.
 13. (canceled)
 14. A method according to claim 11 wherein the rivet has a bore which runs through the tip and through at least a portion of the shank, thereby providing a substantially tubular shank portion with a radially inner shank surface.
 15. A method according to claim 14 wherein the tip is provided by substantially longitudinal slots in the substantially tubular portion.
 16. A method according to claim 15, wherein the top is circumferentially discontinuous across its entire radial extent and wherein the slots extend between the radially inner shank surface and the radially outer shank surface.
 17. (canceled)
 18. A method according to claim 11 wherein the tip has an external taper surface which intersects the radially outer shank surface. 19-29. (canceled)
 30. A method of inserting a rivet into a workpiece using a riveting tool, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: wherein the riveting tool comprises a tool nose and a punch reciprocally disposed therein; the punch provides axial force to the rivet so as to drive it into the workpiece; the rivet is rotated about its longitudinal axis, relative to the workpiece, by the nose of the riveting tool for at least part of the time during which it is in contact with the workpiece; and the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete.
 31. A method according to claim 30 wherein the punch does or does not rotate relative to the workpiece during driving of the rivet into the workpiece 32-36. (canceled)
 37. A method of inserting a rivet into a workpiece, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: the rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece; the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete; and the rivet has a longitudinal bore which extends along its entire axial length.
 38. A method according to claim 37 wherein the rivet is rotated by a rotary drive component of a riveting tool, the rotary drive component engaging with a section of the bore which has a non-circular cross section.
 39. A method according to claim 38 wherein the rotary drive component is a punch which provides axial force to the rivet so as to drive it into the workpiece, the punch engaging with said section of the bore through a complementarily-shaped driving bit projecting therefrom.
 40. A method according to claim 39 wherein the driving bit is movable between an extended position in which it projects from a distal surface of the punch, to a retracted position in which it projects from said distal surface of the punch to a reduced extent or is flush with said distal surface of the punch.
 41. (canceled)
 42. A method according to claim 39 wherein punch has profiled tip which applies said axial force to one axial end of the rivet, and deforms that end of the rivet during driving of the rivet into the workpiece.
 43. A method of inserting a rivet into a workpiece, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: the rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece; the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete; the rivet has a tip for piercing the workpiece at one axial end, a shank extending longitudinally from the tip, and a head extending radially outwards from the shank; the head defines an underside which faces towards the tip; and the rivet has a cavity provided in the underside of the head, or in a portion of the shank adjacent thereto, within which workpiece material may be accommodated. 44-47. (canceled)
 48. A method of inserting a rivet into a workpiece, the method comprising moving the rivet and workpiece relative to one another, along a longitudinal axis of the rivet, so as to drive the rivet into the workpiece, wherein: the rivet is rotated about its longitudinal axis, relative to the workpiece, for at least part of the time during which it is in contact with the workpiece; the speed of said rotation, or the speed of movement along the longitudinal axis of the rivet, is altered at least once before driving of the rivet into the workpiece is complete; and auxiliary heating is applied to the workpiece and/or the rivet at at least one point before, during or after driving of the rivet into the workpiece. 49-57. (canceled)
 58. A method according to claim 48 wherein the rivet penetrates at least a portion of the workpiece with a rotational speed of substantially zero. 59-60. (canceled)
 61. A method according to claim 48 wherein the or at least one of the alterations in axial or rotational speed of the rivet are brought about by resistive forces applied to the rivet by the workpiece.
 62. (canceled)
 63. A method according to claim 48 wherein the or at least one of the alterations in axial speed of the rivet are brought about by an alteration in the magnitude of axial force applied to the rivet or workpiece so as to drive the rivet into the workpiece. 64-65. (canceled)
 66. A method according to claim 48 wherein the axial movement of the rivet relative to the workpiece is paused at at least one point before driving of the rivet into the workpiece is complete.
 67. (canceled)
 68. A method according to claim 48 wherein the axial movement of the rivet relative to the workpiece is paused when the rivet first contacts the workpiece. 69-71. (canceled) 